rear end (differential) gears ?

[Insidious Surmiser]

Is there any way to tell what ratio they are w/ out taking them out :dunno:

 

[starscream5000]

Has there been an engine swap in your car? what year is it? turbo or na?

 

[Insidious Surmiser]

here's why i ask

it's a 89 turbo
it now has a jdm engine in it that i worked on
previous engine is believed to be the original, and the tranny (i think, never really checked) is a r-154

the tach seems to be off now, because i hit rev limit @ 6,500 rpms (on tach)
speedo seems to be about 5-7 mph fast
reads 3,100 rpm @80

 

[suprahero]

3100 sounds about perfect. That's where my five speed runs at 80mph.

 

[Insidious Surmiser]

and you have the 3.7x rear end?

yeh, i forgot the last number :-/

 

[ma71supraturbo]

From http://www.supramania.com/forums/showthread.php?t=32898


But, if you did change your rear end, your speedometer would still indicate 80mph at 3100rpm (because the speedometer cable attaches to the transmission -- so it has no idea what rear end is in the car)

 

[gtsfirefighter]

From http://www.supramania.com/forums/showthread.php?t=32898


But, if you did change your rear end, your speedometer would still indicate 80mph at 3100rpm (because the speedometer cable attaches to the transmission -- so it has no idea what rear end is in the car)

Umm, I don't think that's true. My dad has my g-fathers 76 F150 and the rear end was changed in it years and years ago and since then the speedo has been considerably off. Reads much faster than what you doing. I may be comparing apples to oranges and if so I'm sure I'll be corrected. My question is if you have a jdm auto tranny swap would that affect speedo? Mine is 10-mph fast and has a jdm auto.

 

[ma71supraturbo]

Umm, I don't think that's true. My dad has my g-fathers 76 F150 and the rear end was changed in it years and years ago and since then the speedo has been considerably off. Reads much faster than what you doing.

Your speedometer will read the same mph it did before at the same rpm. So whenever you're at 3100rpm in 5th gear, it will say you're going 80mph. You could be actually going 85, or 75, but your speedometer will read the same mph/rpm as it always did.


As for your jdm auto, it sounds like the car you got the automatic from had a different rear end (so your speedometer gear is off).

 

[stratoayu]

also remember that if u have aftermarket tires, your speedo will be affected if you went over/under the stock tire size..

 

[starscream5000]

Have you changed tires/rims recently too?

 

[gtsfirefighter]

No, stock sawblades with the right size tires.

 

[Insidious Surmiser]

it's the same r-154 that was in the car when i got it... the tires and wheels are original size as well

 

[gtsfirefighter]

Do 7mgte's have the same automatic transmission as the 7mge's? If not I'm wondering if my tranny came from a turbo model.

 

[ma71supraturbo]

gear-ratio wise, yes they are the same. There may be some internal differences (but I don't know)

 

[tmhutch]

Jack the rear end up and climb underneath. Put a mark on the tire and marks on the pinion yoke and differential housing. Slowly spin the tire 1 rotation while taking note of how many times the pinion/driveshaft turns. If the pinion spins 3 ¾ times to one rotation of the tire than you have a 3.73 rear gear. If it spins slightly less than 4 ¼ than you have a 4.11 etc.

Note: this only works on limited slip/positraction type year ends where both rear wheels are spinning at the same time while you rotate them.

Hope that helps,

Todd